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Public Key Cryptography's Impact on Society: How Diffie and Hellman Changed the World

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Public Key Cryptography's Impact on Society: How Diffie and Hellman Changed the World Public Key Cryptography’s Impact on Society: How Diffie and Hellman Changed the World Paul C. van Oorschot∗ Abstract. In 1975 and 1976, Whitfield Diffie and Martin Hellman conceived and intro- duced fundamental new methods that changed how communications are secured. Their landmark paper “New Directions in Cryptography” explained both public key cryptography and what would become known as Diffie-Hellman key exchange. These ideas, influenced and augmented by a few souls within a small community, set the world on a new course by establishing novel cryptographic techniques for protecting information transmitted over untrusted channels. Our aim herein is to consider how public key cryptography has changed the world, and in particular its impact on society. We review the original contributions of Diffie and Hellman, and provide context to relate these to pre-existing and subsequent cryptographic techniques. Aided by this understanding, we connect their contributions to resulting major changes in society. To retain accessibility for non-specialists, our treat- ment largely avoids mathematical details, while selectively introducing technical terms to maintain technical accuracy. 1 Security background We begin with some basic concepts and terminology to develop a working vocabulary. When information is transmitted over a physical channel (physical line) such as a traditional phone line, cable, or optical fibre, the line may be physically shielded or isolated, to reduce the risk of unauthorized access such as by a physical wiretap. If such a communication channel is accessible to unintended parties, it is called an open or untrusted channel. In general, ordinary information (plaintext) sent over untrusted channels is at risk of interception. For example, plaintext sent over a radio channel is accessible to anyone with a suitable wireless receiver. A common defense is to convert plaintext characters into a related sequence of characters (ciphertext) that are not meaningful even if intercepted. To do so, at the sender’s end a sequence of instructions (called an encryption algorithm) is used to convert plaintext to ciphertext, which is then transmitted. To recover the plaintext, the operation is reversed at the receiver’s end by a decryption algorithm. In this way, encryption provides a confidentiality property, whereby the meaningful con- tent is available only to authorized parties. Unauthorized parties cannot recover the plain- text because the encryption and decryption algorithms require a secret number, which may be viewed as a random string of 0s and 1s; 128 of these would be called a 128-bit crypto- graphic key. The aim is that only the sender and recipient (i.e., their computing devices) share this secret key.1 Historically, decryption requires the same key as used for encryption; in this case we use the terms symmetric-key algorithms and symmetric keys. Distinct from confidentiality or secrecy is the concept of authentication. The ability to recognize individuals (entity authentication) is taken for granted in human-to-human interactions, but more challenging in written communications. (Can you be sure that the postcard you have received is legitimately from your sister?) To provide a modest degree of authentication beyond relying solely on context and semantic content, we conventionally ∗14 April 2020. A version of this paper is to appear in a forthcoming ACM book. 1Viewing the encryption process as an algorithm, the key is a required input parameter. For a given plaintext message, a different key results in a different ciphertext. 1 end handwritten letters with personal signatures. Historically, various alternatives have been used to provide arguably stronger assurances. One example is wax seals created using recognizable signet rings, in some cases affixed alongside a handwritten signature, and in other cases on an exterior envelope, thereby providing also a signal in case the envelope has been opened and/or the content altered. Such means serve to prevent or detect alterations, also called integrity violations. Mechanisms that convey assurances regarding the source of transmitted content are said to provide data origin authentication; in practice, such mechanisms also provide integrity protection, i.e., are able to detect if content has been altered. In dealing with computer data, it is helpful to define data authentication as the combination of data origin authentication and data integrity. We then try to avoid the ambiguous term authentication, instead using entity authentication or data authentication. 2 Context: motivation and environment The invention of public key cryptography resulted from a combination of elements. These included the inventors’ personal perspectives and motivation, and an environment that led to key management and data authentication being recognized as important open problems. The need for digital signatures was a seed planted in 1970. While working on artificial intelligence research for John McCarthy at Stanford, Diffie was exposed to McCarthy’s early interest in “buying and selling through home terminals” [68]. What we now call electronic commerce (e-commerce) was a new idea in this age before personal computers, when even home-based terminal connections to workplaces were rare. This led Diffie to recognize an open question [18, 88]: How would one authenticate digital transactions with remote parties, in lieu of assurances provided by handwritten signatures on paper contracts? Known techniques [18] for authentication at that time, using a symmetric key shared between buyer and seller would fail if one party originated a message but then denied this, claiming that the other party had done so. Indeed, parties in business and consumer transactions often have competing interests, while business partners on one project may compete on the next. Moving now to encrypted communication between two remote parties, this convention- ally required that a symmetric key be shared between them, prior to the beginning of the secure communication. The key is not sent over the same channel as the message—if that channel itself provided confidentiality, encryption would not be needed for the mes- sage. Thus conventionally, it was necessary to (1) know, ahead of time, who you planned to communicate with securely; and (2) pre-distribute a secret key to them over a trusted channel—thus the stereotypical briefcase chained to a courier’s wrist. This is of course costly in both time and money. However in specific environments, such as the military and some government organizations, it is feasible due to, e.g., centralized organization with hierarchi- cal reporting trees, and relatively static, well-known chains of command. Nevertheless, such pre-distribution of keys is particularly challenging at scale; according to Diffie, at one time the US National Security Agency (NSA) reportedly had “the biggest printing press in the world at Fort Meade” [17], used at least in part for printing keying material. In May 1973, the US government’s National Bureau of Standards (NBS, the precursor of NIST) solicited proposals, for “algorithms for the encryption of computer data to ensure their protection during transmission over exposed communications facilities or while recorded on media in transport or in storage” [60]. Following a second call in August 1974 [61], IBM submitted an algorithm that would become the Data Encryption Standard (DES), after its formal proposal by NBS in March 1975 [62]. In November 1976, DES was approved as a Federal Information Processing Standard (FIPS), FIPS 46. This DES-related activity opened, for the first time, the possibility of broad use of encryption by the general public. In parallel, Hellman and Diffie, who had met in September 1974 [68], recognized key management as a barrier [18] to broad adoption of encrypted communications. They noted: if there are n users, for each to share a distinct key with each of n − 1 others would require n(n − 1)=2 or roughly n2 keys across the system. This number grows quickly as n increases. Pre-distribution of this many keys, e.g., by physical 2 courier, would be impractical across the general public—due to both the number of users, and absence of the simplifying structures noted above. As a separate issue, centralized key distribution was not an appealing solution for indi- viduals who preferred not to rely on, or place trust in, governments and large organizations. Such individuals included Diffie, “who grew up in a libertarian and leftist tradition” [84]. In Diffie’s words, “I had a very counter-cultural viewpoint and it was very anti-central author- ity” [88]. His interest in key management dated back to 1965 when he mistakenly2 came to believe that for within-facility calls, NSA phone lines were encrypted, using keys distributed by a central facility [88]. He didn’t see the point of such an architecture—his view was that cryptography should reduce the number of parties that one had to trust. Thus two com- municating parties should use keying material known to them alone. A key management solution that required trusting a central facility missed the mark, leaving key management as an open problem in his mind. In the fall of 1974, Diffie also first learned of the NBS plan to standardize encryption from Butler Lampson [88]. Believing that the US government might not standardize an algorithm unless they themselves could break it, Diffie began to ponder whether somehow a “trap-door” might be built into the algorithm, such that a party with special knowledge of the trap-door (but no one else) could easily break the algorithm—an idea that he credits [88] for inspiring public key cryptography. Thus the DES activity may be seen to have motivated the inventive contributions of Diffie and Hellman both through its heightening the need for key management, and by triggering the idea of trap-door cryptosystems. As we explain next, public key encryption, and Diffie-Hellman key exchange, provided two different solutions to the key management problem. Public key cryptography also provided a solution to the open problem of digital-world signatures.
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